Tribology is the science of lubrication, friction and wear in objects which rub against each other in motion.
A tribological system is the collection of four main parts. They are, the main body which moves, the opposing body which resists the motion, the intermediate material and the surrouding medium.
Consider a shaft supported by a bearing. In this example, the main body is the shaft, the opposing body is the bearing shell, the intermediate material is the grease and the surrouding medium are the other components on which the bearing and the shaft are mounted.
Tribological systems are classified into open and closed systems.
Bearings and seals are examples of closed tribological systems
Examples of open tribological systems are channels, pipes, etc. In these system, the intermediate material acts as the opposing body.